(1) Field of the Invention
This invention relates to improvements in and relating to a screen replacing mechanism for extruders.
(2) Description of the Prior Art
In some cases, an extruder is used in combination with a pelletizer which is located at the top end of the extruder for the purpose of pelletizing a thermoplastic synthetic resin or the like.
With regard to the pelletizing operation, there have thus far been proposed various types of pelletizers, of which the most typical is the underwater pelletizer which is accepted in many cases for its high pelletizing efficiency.
The construction of the underwater pelletizer is well known in the art so that particulars of its construction are omitted herein. When starting a pelletizing operation by the use of such a pelletizer, it is the general practice to actuate the extruder in advance to send forward a plasticized resin material by an extruding screw in a cylinder through a passage (nozzle) in a die plate at the fore end of the cylinder confirming whether or not the resin material is extruded at a uniform or nonuniform speed from the passage in the die plate. At this time, the resin passages in the cylinder and die plate are filled with the plasticized resin material. After the preparatory operation, the extruder is once stopped to clean off the plasticized resin which has deposited on the surface of the die plate and restarted to commence the pelletizing operation. During the temporary stop of the extruder, the extruding material flows out of the die plate nozzle and deposits on the cleaned surfaces of the die plate under the influence of the residual pressure which lingers in the extruder due to the viscosity of the plasticized material and the fluidity of the extruding material which is heated and plasticized by the extruder for extrusion.
If the extruder is restarted to initiate the pelletizing operation with the deposition of the resin material on the surfaces of the die plate, difficulty is encountered in cutting the extrudate by a rotating cutter knife of the pelletizer, the deposit of a plasticized resin on the die plate sticking on the cutter knife without undergoing its cutting action which is necessary for the pelletizing. Further, when restarting the extruder, it has been necessary to increase the rotational speed of the extruding screw with appropriate timing for elevating the speed of extrusion through the die plate, and at the same time supplying cooling water to a water chamber of the pelletizer also with appropriate timing. Such operation requires a high level technique and meticulous skill. Nevertheless, even if an operator has succeeded in performing these operations, it has been extremely difficult to keep the operation constantly at a stable state.
Although the foregoing description has been directed to a underwater pelletizer, it is necessary in almost all types of extruders including the underwater pelletizer to remove residual extruding material from the cylinder in advance by actuating the extruder after removing the extruding head or a screen assembly at the fore end of the cylinder at the time of cleaning the extruding cylinder or changing the extruding material or its color.